The present invention relates to a SQUID (superconducting quantum interference device) used for a magnetic sensor for measuring a biomagnetic field, and more particularly, to a technique suitable for a case where a SQUID and pickup coils are formed by using a high-Tc superconductor having superconductive characteristics in liquid nitrogen.
SQUID magnetometers used for measuring in the biomagnetic field are described in, for example, "Review of Scientific Instrument", Vol. 53, No. 12, pp. 1815-1845. The SQUID described in the literature is constructed by using a well-known metal superconductor using niobium (Nb), lead (Pb), or alloys there of as a main component and operates in liquid helium (4.2K) together with pickup coils. A SQUID using what is called a high-Tc superconductor made of YBa.sub.2 CU.sub.3 O.sub.y (YBCO) or the like which becomes superconductive in liquid nitrogen (77K) has been developed. Since the liquid nitrogen that is cheap and handled easily can be used as a cooling agent, the high-Tc SQUID is extremely useful for wide use as a biomagnetic field measuring device. In recent years, since the performance of the high-temperature SQUID is improved, the biomagnetic field can be also measured by a fluxmeter using the high-temperature SQUID.
Examples of the high-temperature SQUID fluxmeter are described in "Applied Physics Letter", Vol. 68, No. 10, pp. 1421-1423 (hereinbelow, referred to as a first example), "IEEE Transaction of Superconductivity", Vol. 5, No. 2, pp. 2919-2922 (hereinbelow, called a second example), "IEEE Transaction of Superconductivity", Vol. 5, No. 2, pp. 2927-2930 (hereinbelow, called a third example), "Applied Physics Letter", Vol. 63, No. 16, pp 2271-2273 (hereinbelow, called a fourth example), and the like.
Like all of the first to fourth examples, a dc-SQUID, that is, a superconductive ring having two Josephson junctions is considered as the SQUID here. FIGS. 10 and 11 show current versus voltage characteristics and flux to voltage conversion characteristics of a well-known SQUID. In FIG. 10, .PHI..sub.ex is a magnetic flux applied to the SQUID, .PHI..sub.o is fluxoid quantum, Ib is a bias current value giving the flux to voltage conversion characteristics, and n is an integer. FIG. 10 shows a state such that a step occurs when .PHI..sub.ex is (n+1/2).PHI..sub.o due to resonance of a microwave generated depending on the ring shape of the SQUID. It is known that a voltage amplitude .DELTA.V is regulated due to the step.
In a manner similar to a metal SQUID, the high-temperature SQUID is formed on a substrate. As shown in the first to fourth examples, the detection coil is also generally formed on the same substrate. One of reasons for the construction is that it is difficult to join high-temperature superconductors as compared with a metal superconductor and it is cult to separately fabricate a SQUID and a detection coil and join them in a manner similar to a fluxmeter using a metal superconductor. It is not always easy to form high-temperature superconductive multilayer films on a substrate. If both of the SQUID and the detection coil can be formed by a single high-temperature superconductor layer as shown in the third and fourth examples, the fabrication becomes much easier and the yield is also improved.
The SQUID fluxmeter in the third or fourth example is called a direct-coupling SQUID. FIG. 5 shows a configuration of the fluxmeter and FIG. 6 shows an example of a construction of a substrate on which the SQUID and the detection coil are mounted. FIG. 6a corresponds to a part 10 surrounded by a dotted line in FIG. 5. A hatched part is a superconductive film part and the rest is a part the substrate is exposed. FIG. 6b is an enlarged diagram of a SQUID part (*1) surrounded by a circle in FIG. 6a. A SQUID ring is a superconductive ring comprising two Josephson junctions 1 and superconductors 2, 2' for connecting the Josephson junctions 1. A direct-coupling SQUID has a construction that a detection coil 3 is parallelly connected to the supercoductor 2 constructing the SQUID ring. The Josephson junctions of the high-temperature SQUID are often formed in a bicrystal part (discontinuous part in a crystal direction) of a substrate represented by SrTiO.sub.3 or a step edge part (step part). Since a bicrystal line 5 is generally positioned in the center of the substrate, the example of FIG. 6 has a construction that the SQUID and the detection coil are asymmetric. In FIG. 6a, reference numeral 6 denotes a pad for connection. In FIG. 5, reference numeral 20 denotes an amplifier, reference numeral 30 denotes a Flux-Locked-Loop circuit, and reference numeral 40 denotes a bias current source.
In the direct-coupling,SQUID, as described in the third example, when a noise .PHI..sub.n the fluxmeter is a flux noise of a SQUID input conversion a magnetic noise of a detection coil input conversion corresponding to sensitivity of the fluxmeter is shown by .PHI.n/A.sub.eff. A.sub.eff denotes a magnetic field detecting area of the detection coil which is approximately given by A.sub.p (L.sub.c /L.sub.p) where A.sub.p is an effective area of the pickup coil, L.sub.p is an inductance of the pickup coil, L.sub.c is a component contributing the magnetic coupling with pickup coil among all of inductances Ls of the SQUID ring, and L.sub.c &lt;L.sub.s. It is known that the flux noise .PHI..sub.n of the SQUID input conversion is reduced as the voltage modulation .DELTA.V of the flux to voltage conversion characteristics of the SQUID becomes larger.